Ebola virus continues its rampage in West Africa. There were 1603 known cases as of last week, and there is no sign of the epidemic slowing. More than half of those infected have died.
Every infection gives Ebola a chance to better adapt to humans, and this outbreak is nearly four times as large as the previous largest. But despite what you may have seen in lurid headlines, Ebola so far shows no sign of evolving into a global pandemic. The factor that could make this virus a bigger killer is social: urbanisation, the same thing that unleashed HIV.
Global concerns have focused on whether the virus could spread beyond the three chiefly affected countries – Guinea, Liberia and Sierra Leone. An infected traveller has spread the virus to three others in Nigeria, and as New Scientist went to press there were unconfirmed reports of potentially infected travellers in Morocco and Saudi Arabia. The World Health Organization is meeting this week to decide if the outbreak is an international public health emergency.
Whatever the organisation decides, it should be possible to contain the outbreak. The virus only spreads from people with dramatic symptoms, says Andrew Read of Penn State University in University Park, making it easy to spot cases and isolate those who have come into contact with an infected person. Because the virus can incubate for 21 days, the worry is that it may be missed if travellers get sick after arrival. That said, the risk is small as catching Ebola is relatively hard, requiring exposure to body fluids.
Another concern is that the virus could evolve into a more transmissible form. So far, there are no signs of this happening. The pressure the virus faces to successfully replicate in humans, says Andrew Rambaut of the University of Edinburgh, UK, is far greater than the pressure to transmit more readily.
Taken together, this is why a large outbreak in a developed country is unlikely. But is the current outbreak the result of a new invasion of the virus? There have been no recorded cases of Ebola in this part of West Africa before, but Daniel Bausch of Tulane University in New Orleans, Louisiana, thinks it may have been present but unrecognised.
Previous outbreaks have been in central Africa, where the virus is thought to be carried by fruit bats, and the same bats live in the affected part of West Africa. It might not have been recognised because only a minority of infected people exhibit the haemorrhaging for which the virus is known. Most get flu-like symptoms, then fever, vomiting and diarrhoea, and die from fluid loss (see “Anatomy of a killer“). These symptoms resemble those of other local diseases such as malaria and Lassa fever. Researchers are retesting serum samples from Lassa surveys in the 1990s for signs of Ebola.
It’s the magnitude of this outbreak that will make it hard to control, say epidemiologists. “As the number of cases goes up it will become harder to do effective contact tracing,” says Rambaut, so the rate of spread could rise.
So what has made this outbreak so big? The overriding factor could be urbanisation. In the past, village outbreaks remained small, unless people went to hospitals. “Population size and high mobility make it hard to do contact tracing,” says Peter Walsh at the University of Cambridge. Cities provide more chances to spread the virus, something that may also have enabled the spread of HIV. According to the African Development Bank, the continent has had the world’s highest urban growth rate for 20 years, and the proportion of Africans living in cities will rise from 36 per cent to 60 per cent by 2050.
“Cities provide more chances to spread Ebola, much as they helped unleash HIV”
Other factors also favour the virus. Justin Masumu of the National Institute for Biomedical Research in Kinshasa, Democratic Republic of the Congo, found that the increase in Ebola outbreaks since 1994 is associated with changes in forest ecosystems due to deforestation, which displaces bats. The part of Guinea where this outbreak started has been largely deforested.
What’s more, wars in Liberia and Sierra Leone, and corruption in Guinea, have caused poverty, says Bausch, leading people to migrate for work and spread the virus further. It has also caused widespread mistrust of officials, even in public health – just when Africa’s cities need them most.
Anatomy of a killer
There are five strains of the Ebola virus, four of which are deadly to people. The most lethal one, Zaire ebolavirus, is currently infecting people in West Africa (see map above).
The virus is thought to be carried by fruit bats, and people can catch it if they are bitten by, or come into contact with bodily fluids of other species that have been bitten. The disease can also be caught by eating inadequately cooked infected meat.
Once in the blood, the virus stimulates white blood cells to produce an excess of cytokines – chemical messengers of the immune system. This “cytokine storm” weakens the endothelial cells that line blood vessel walls. The blood vessels leak, and blood pressure drops, triggering a state of shock.
If infected, you experience a sudden onset of fever, muscle pain, weakness, headaches, a sore throat, vomiting and diarrhoea. As the infection takes hold, kidney and liver function is impaired and internal and external bleeding can begin. Blood and other fluids carry the virus.
There is currently no cure (see “The long battle to find a cure for Ebola”), but people can recover if they are given intravenous fluids to combat the dehydration caused by bleeding, vomiting and diarrhoea early enough. Philippa Skett
The long battle to find a cure for Ebola
PROGRESS to combat Ebola pharmaceutically has been painfully slow. Several potential drugs and vaccines are working their way through animal studies and clinical trials, but on-the-ground trials are hard to conduct because outbreaks are so unpredictable.
Last week, the clinical process was sidestepped when two US missionary workers infected with Ebola received an experimental drug called ZMapp. In an unusual step, Kent Brantly and Nancy Writebol were given the drug, which was flown from the US to Liberia. ZMapp is a blend of three monoclonal antibodies which had previously only been tested in infected monkeys. The drug prevents the virus from entering and infecting cells, and reportedly stabilised both enough that they could fly home for further treatment.
The only Ebola treatment to have reached human trials uses RNA interference. The approach involves using RNA molecules to stop DNA from making proteins, and thus stop the virus from replicating. The drug, called TKM-Ebola, protected monkeys when it was given to them within 30 minutes of being injected with the virus. Safety studies in humans have been paused, however, while the manufacturer gets more data to the US Food and Drug Administration about how the immune system responds to high doses.
Even better would be a vaccine. The US National Institutes of Health announced last week that it will speed up testing a vaccine based on a harmless adenovirus. It could enter human trials later this year and be available next. The adenovirus is genetically altered to produce two Ebola proteins. The immune system of a vaccinated person recognises these proteins and produces antibodies to them if it encounters a real Ebola virus. The vaccine could also be used to prevent illness in an infected person if given early enough.